Christ. That's fucking dark. Where the hell did you get this idea? Obviously she remembered but was too embarrassed to say. Just look at the way that scene is framed. There's a reason her face isn't shown when she's saying that line. Also, Kaya obviously knows that Sana does remember and is just putting on a front. Look at her face!
Maybe flipping the framing would have allayed reader worries, showing Sana facing away while blushing would have made it obvious she remembers. However, we only follow Kaya's POV and empathize primarily with her, so I believe we are expected to feel her insecurity and react accordingly. In this regard, I would say the story was a success considering the reactions in this forum.
They aren't stuck in a toxic failure of a relationship, they just have normal problems with communication, the kind that exist in every relationship.
While I agree that this is not a toxic relationship per se, they have serious communication issues; I mean, as far as we know, Kaya hasn't received a confirmation of their relationship in the 5 years they have been together.
We were shown the snap that forces them to either break up or fix some of their issues, but, because of those very problems, Sana is grasping at straws trying to understand why her girlfriend snapped, and thus goes for the only thing different (denying remembering their first movie night) to hopefully appease her.
If they don't talk this through, then their relationship is likely to start becoming toxic, with Kaya becoming increasingly bitter and Sana doing small things trying to keep the status quo.